This invention relates to a novel positive photosensitive resin composition which may be employed for preparation of a printed wiring board or the like. More particularly, it relates to a novel positive photosensitive resin composition which exhibits superior long-term stability of an electrodepositing solution and which may be deposited on a substrate for forming a superior coating film thereon.
In keeping up with the recent development of electronic equipments, high density, higher integration and fine circuit designing are demanded of a printed circuit board as a support member. Consequently, for mounting components on a substrate, a surface mount system in which through-holes of smaller diameter are bored in the substrate for electrical conduction across the front and back sides of the substrate has been accepted as a mounting system. The through-holes have the inner wall surfaces plated with metal for interconnecting the circuits on the front and back sides of the substrate and for securing the components mounted on the substrate.
When preparing such printed circuit boards having these through-holes and fine interconnects, it is necessary to protect even the inner wall surfaces of the through-holes from an etching solution. However, with a method of laminating a photosensitive film, known as a dry film, on the substrate surface, which is a method employed nowadays extensively, since the film itself generally has a larger thickness of the order of 50 .mu.m, a sharp circuit pattern cannot be produced on exposure to light and development. In addition, difficulties are presented in laminating the dry film uniformly on the metal surface. Above all, the through-holes with a smaller diameter cannot be coated with the photosensitive film.
If with a method of uniformly applying an etching resist by electrodeposition, a conventional negative electrodeposition photoresist, consisting essentially of a photocurable resin, is employed, a sufficient amount of light cannot be radiated into the inside of the through-hole so that the resin cannot be cured sufficiently within the through-hole and hence it is difficult to protect the electrically conductive layer within the inner wall of the through-hole against the etching solution. For overcoming such deficiency, investigations are currently conducted towards developing a positive photoresist in which an unexposed portion serves as a protective layer. Besides, investigations are also conducted towards developing a positive electrdepositive photoresist which may be applied uniformly on the substrate by electrodeposition coating.
With the use of the positive photoresist, since a resist portion irradiated with light is dissolved in a developing solution, while an unirradiated resist portion, which is insoluble in the developing solution, is left as a protective layer without being removed, it becomes possible to form a protective layer without the necessity of irradiating the inner surfaces of the through-holes with light.
While a number of functional groups have been proposed as photosensitive groups capable of affording these properties to the positive resist, it is a quinone diazide group that is most excellent in sensitivity and resolution. Such difference in solubility between the exposed portion and the non-exposed portion of the positive photoresist, which is correlated with difference in photosensitivity, may be realized when dissolution promoting groups and the quinone diazide groups are present at a suitable ratio in the resist. The positive photoresist exhibits high photosensitivity if these two functional groups are contained in one and the same resin or in separate resins. However, the resin having both groups contained therein is not desirable from the viewpoint of industry in that it is difficult to purify despite superior stability of the electrodeposition solution. If a compound containing quinone diazide groups and a resin containing dissolution promoting groups, that is an alkali-soluble resin, are mixed together at a moderate ratio to give a positive photoresist, it is crucial to improve compatibility of the two components to realize high resolution and stability of the electrodeposition solution.
Thus, an alkali-soluble resin having one of dicarboxylic acid groups esterified has been proposed in Japanese Laid-Open Patent Application No. 4-96067 (1992). However, the long-term stability of the electrodeposition solution stored at room temperature for six months is lowered due to changes in compatibility between the compound containing quinone diazide groups which are oleophilic and the resin containing dissolution promoting groups, thus lowering the resolution. Besides, the alkali-soluble resin undergoes hydrolysis to adversely affect developability of the solution.